With uni-directional printing, as is conventional in single-pen ink-jet printers, a first print pass of the printhead carriage would proceed from the left to the right, and the carriage would retrace, or return without printing, to the left to the start of a next print pass. Another print pass then would print from the left to the right, and the carriage would retrace to the left to the start of yet another print pass. If such printing method were used in two-pen ink-jet printers, a first color pass, followed by a non-printing retrace, typically followed by a black pass, followed typically by second and third color passes would be required. Thus, a typical printable swath involving tri-color and black ink deposition would take three color print passes, one black print pass and four (non-printing) retraces. Much wasted motion would inhere in such tri-color and black ink-jet printing.
The advent of low-cost two-pen (one black and one tri-color) ink-jet printers thus poses both challenges and problems regarding bidirectional printing. One such bidirectional printing system is described in U.S. Pat. No. 5,044,796, entitled "Bidirectional Printing Method in Accordance with Vertical Breaks" issued Sep. 3, 1991 to Lund, which is subject to common ownership herewith. The disclosure of that patent is incorporated herein by this reference. Special concerns for monochromatic, e.g. black, and tri-color, e.g. cyan, magenta and yellow, print quality and black-to-color liquid ink bleeding phenomena must be addressed in order to maintain high overall print quality. At the same time, it is always desired to increase printer throughput, or at least to maintain the high throughput standards of previous single-pen ink-jet printers. While primitive or primary color blending is unpredictable, as it is largely user- and use-specific, monochrome printing is relatively predictable and thus is more controllable to the potential advantage of many users in varied applications.